1. Technical Field of the Invention
The present invention relates to a double-stator motor having a configuration in which a first three-phase stator is arranged radially inside a rotor and a second three-phase stator is arranged radially outside the rotor.
2. Related Art
Several types of motors, such as motor-generators and in-wheel motors, have been known. A motor-generator is disposed between an engine and a transmission of a vehicle. An in-wheel motor is incorporated into a drive wheel to directly drive the drive wheel. These motors have suffered from installation constraints that are ascribed to the purposes of their use. In other words, due to the limited space of installation, these motors have been desired to have a small axial dimension, or a thin shape.
In order to cope with this issue, double-stator motors have been suggested in which stators are arranged inside and outside a rotor.
For example, JP-A-H03-139156 or JP-A-2007-251342 discloses such a double-stator motor. As can be seen in a model illustrated in FIG. 1A, the double-stator motor as disclosed in these documents includes an inner stator 100, an outer stator 110, a rotor core 120 arranged between the inner and outer stators 100 and 110 to face the inner and outer stators 100 and 110, and permanent magnets 130 and 140 provided at inner and outer surfaces, respectively, of the rotor core 120.
In this type of double-stator motor, magnetic fields are formed by the permanent magnets 130 and 140. Meanwhile, winding magnetomotive force (i.e. in ampere turns) is caused at both of the inner and outer stators 100 and 110. The winding magnetomotive force is added in series to the magnetic fields as indicated by the arrows in FIG. 1A to work on each rotor pole consisting of the two inner and outer permanent magnets 130 and 140. Therefore, the inner and outer stators 100 and 110 are able to generate large torque if the laminate thickness of each of the stators is small. In other words, this type of double-stator motor is able to reduce the laminate core thickness of stators, which is required to obtain predetermined target torque characteristics.
FIG. 1B is a contour diagram analyzing the magnetic flux flow in the double-stator motor shown in FIG. 1A.
However, well-known double-stator motors such as the one mentioned above have the following problems (1) to (3) which make it difficult to use these motors in practice.
(1) Fabrication cost is high because of the use of a lot of strong magnets.
(2) The output is small in relation to the quantity of magnets in use and to the size and volume of the motor.
(3) The efficiency is poor in high-speed rotation, such as in high-speed cruising.
As a result of the analysis and study of the above problems (1) to (3) by the inventors of the present invention, the following underlying causes have been found.
The reason why a lot of strong magnets are needed is that, in a narrow space formed between inner and outer stators, magnets are required to supply sufficient magnetic flux to each stator and required to have demagnetization strength against the demagnetizing fields loaded from the two series stator winding ampere turn.
The reason why a large output cannot be obtained in relation to the quantity of magnets and to the size and volume of the motor is that the cores are saturated and thus magnetic flux leakage is increased. Specifically, magnetic flux is passed through the cores of the inner and outer stators and two gaps (four gaps regarding a magnetic flux loop of one pole pair). Therefore, magnetic flux will have a long path and the magnetic resistance will be increased accordingly, leading to the tendency of magnetic flux leakage. Due to the large magnetic resistance, magnets having very strong magnetomotive force are required to be arranged to thereby pass desired magnetic flux. Accordingly, the difference in magnetic potential between portions of the magnetic circuit will become large, further accelerating the tendency of magnetic flux leakage.
The reason why the efficiency is poor in high-speed rotation is that the cores are likely to be saturated, as mentioned above, and thus eddy current loss and hysteresis loss are increased, and, being dependent on the frequency, core loss is increased.